Miniature telephoto lens assembly

ABSTRACT

An optical lens assembly includes five lens elements and provides a TTL/EFL&lt;1.0. In an embodiment, the focal length of the first lens element f1&lt;TTL/2, an air gap between first and second lens elements is smaller than half the second lens element thickness, an air gap between the third and fourth lens elements is greater than TTL/5 and an air gap between the fourth and fifth lens elements is smaller than about 1.5 times the fifth lens element thickness. All lens elements may be aspheric.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. patentapplication Ser. No. 15/817,235 filed Nov. 19, 2017, which was aContinuation application of U.S. patent application Ser. No. 15/418,925filed Jan. 30, 2017, now U.S. Pat. No. 9,857,568, which was aContinuation in Part application of U.S. patent application Ser. No.15/170,472 filed Jun. 1, 2016, now U.S. Pat. No. 9,568,712, which was aContinuation application of U.S. patent application Ser. No. 14/932,319filed Nov. 4, 2015, now U.S. Pat. No. 9,402,032, which was aContinuation application of U.S. patent application Ser. No. 14/367,924filed Jun. 22, 2014, now abandoned, which was a 371 Continuationapplication of international application PCT/IB2014/062465 filed Jun.20, 2014, and is related to and claims priority from U.S. ProvisionalPatent Application No. 61/842,987 filed Jul. 4, 2013, which isincorporated herein by reference in its entirety.

FIELD

Embodiments disclosed herein relate to an optical lens system and lensassembly, and more particularly, to a miniature telephoto lens assemblyincluded in such a system and used in a portable electronic product suchas a cellphone.

BACKGROUND

Digital camera modules are currently being incorporated into a varietyof host devices. Such host devices include cellular telephones, personaldata assistants (PDAs), computers, and so forth. Consumer demand fordigital camera modules in host devices continues to grow. Cameras incellphone devices in particular require a compact imaging lens systemfor good quality imaging and with a small total track length (TTL).Conventional lens assemblies comprising four lens elements are no longersufficient for good quality imaging in such devices. The latest lensassembly designs, e.g. as in U.S. Pat. No. 8,395,851, use five lenselements. However, the design in U.S. Pat. No. 8,395,851 suffers from atleast the fact that the TTL/EFL (effective focal length) ratio is toolarge.

Therefore, a need exists in the art for a five lens element optical lensassembly that can provide a small TTL/EFL ratio and better image qualitythan existing lens assemblies.

SUMMARY

Embodiments disclosed herein refer to an optical lens assemblycomprising, in order from an object side to an image side: a first lenselement with positive refractive power having a convex object-sidesurface, a second lens element with negative refractive power having athickness d₂ on an optical axis and separated from the first lenselement by a first air gap, a third lens element with negativerefractive power and separated from the second lens element by a secondair gap, a fourth lens element having a positive refractive power andseparated from the third lens element by a third air gap, and a fifthlens element having a negative refractive power, separated from thefourth lens element by a fourth air gap, the fifth lens element having athickness d₅ on the optical axis.

An optical lens system incorporating the lens assembly may furtherinclude a stop positioned before the first lens element, a glass windowdisposed between the image-side surface of the fifth lens element and animage sensor with an image plane on which an image of the object isformed.

The effective focal length of the lens assembly is marked “EFL” and thetotal track length on an optical axis between the object-side surface ofthe first lens element and the electronic sensor is marked “TTL”. In allembodiments, TTL is smaller than the EFL, i.e. the TTL/EFL ratio issmaller than 1.0. In some embodiments, the TTL/EFL ratio is smaller than0.9. In an embodiment, the TTL/EFL ratio is about 0.85. In allembodiments, the lens assembly has an F number F#<3.2. In an embodiment,the focal length of the first lens element f1 is smaller than TTL/2, thefirst, third and fifth lens elements have each an Abbe number (“Vd”)greater than 50, the second and fourth lens elements have each an Abbenumber smaller than 30, the first air gap is smaller than d₂/2, thethird air gap is greater than TTL/5 and the fourth air gap is smallerthan 1.5d₅. In some embodiments, the surfaces of the lens elements maybe aspheric.

In an optical lens assembly disclosed herein, the first lens elementwith positive refractive power allows the TTL of the lens system to befavorably reduced. The combined design of the first, second and thirdlens elements plus the relative short distances between them enable along EFL and a short TTL. The same combination, together with the highdispersion (low Vd) for the second lens element and low dispersion (highVd) for the first and third lens elements, also helps to reducechromatic aberration. In particular, the ratio TTL/EFL<1.0 and minimalchromatic aberration are obtained by fulfilling the relationship1.2×|f3|>|f2|>1.5×f1, where “f” indicates the lens element effectivefocal length and the numerals 1, 2, 3, 4, 5 indicate the lens elementnumber.

The conditions TTL/EFL<1.0 and F#<3.2 can lead to a large ratio L11/L1 e(e.g. larger than 4) between the largest width (thickness) L11 and thesmallest width (thickness) of the first lens element (facing the object)L1 e. The largest width is along the optical axis and the smallest widthis of a flat circumferential edge of the lens element. L11 and L1 e areshown in each of elements 102, 202 and 302. A large L11/L1 e ratio(e.g. >4) impacts negatively the manufacturability of the lens and itsquality. Advantageously, the present inventors have succeeded indesigning the first lens element to have a L11/L1 e ratio smaller than4, smaller than 3.5, smaller than 3.2, smaller than 3.1 (respectively3.01 for element 102 and 3.08 for element 302) and even smaller than 3.0(2.916 for element 202). The significant reduction in the L11/L1 e ratioimproves the manufacturability and increases the quality of lensassemblies disclosed herein.

The relatively large distance between the third and the fourth lenselements plus the combined design of the fourth and fifth lens elementsassist in bringing all fields' focal points to the image plane. Also,because the fourth and fifth lens elements have different dispersionsand have respectively positive and negative power, they help inminimizing chromatic aberration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a first embodiment of an optical lens system disclosedherein;

FIG. 1B shows the modulus of the optical transfer function (MTF) vs.focus shift of the entire optical lens assembly for various fields inthe first embodiment;

FIG. 1C shows the distortion vs. field angle (+Y direction) in percentin the first embodiment;

FIG. 2A shows a second embodiment of an optical lens system disclosedherein;

FIG. 2B shows the MTF vs. focus shift of the entire optical lensassembly for various fields in the second embodiment;

FIG. 2C shows the distortion +y in percent in the second embodiment;

FIG. 3A shows a third embodiment of an optical lens system disclosedherein;

FIG. 3B shows the MTF vs. focus shift of the entire optical lens systemfor various fields in the third embodiment;

FIG. 3C shows the distortion +Y in percent in the third embodiment.

DETAILED DESCRIPTION

In the following description, the shape (convex or concave) of a lenselement surface is defined as viewed from the respective side (i.e. froman object side or from an image side). FIG. 1A shows a first embodimentof an optical lens system disclosed herein and marked 100. FIG. 1B showsthe MTF vs. focus shift of the entire optical lens system for variousfields in embodiment 100. FIG. 1C shows the distortion +Y in percent vs.field. Embodiment 100 comprises in order from an object side to an imageside: an optional stop 101; a first plastic lens element 102 withpositive refractive power having a convex object-side surface 102 a anda convex or concave image-side surface 102 b; a second plastic lenselement 104 with negative refractive power and having a meniscus convexobject-side surface 104 a, with an image side surface marked 104 b; athird plastic lens element 106 with negative refractive power having aconcave object-side surface 106 a with an inflection point and a concaveimage-side surface 106 b; a fourth plastic lens element 108 withpositive refractive power having a positive meniscus, with a concaveobject-side surface marked 108 a and an image-side surface marked 108 b;and a fifth plastic lens element 110 with negative refractive powerhaving a negative meniscus, with a concave object-side surface marked110 a and an image-side surface marked 110 b. The optical lens systemfurther comprises an optional glass window 112 disposed between theimage-side surface 110 b of fifth lens element 110 and an image plane114 for image formation of an object. Moreover, an image sensor (notshown) is disposed at image plane 114 for the image formation.

In embodiment 100, all lens element surfaces are aspheric. Detailedoptical data is given in Table 1, and the aspheric surface data is givenin Table 2, wherein the units of the radius of curvature (R), lenselement thickness and/or distances between elements along the opticalaxis and diameter are expressed in mm. “Nd” is the refraction index. Theequation of the aspheric surface profiles is expressed by:

$z = {\frac{{cr}^{2}}{1 + \sqrt{1 - {( {1 + k} )c^{2}r^{2}}}} + {\alpha_{1}r^{2}} + {\alpha_{2}r^{4}} + {\alpha_{3}r^{6}} + {\alpha_{4}r^{8}} + {\alpha_{5}r^{10}} + {\alpha_{6}r^{12}} + {\alpha_{7}r^{14}}}$where r is distance from (and perpendicular to) the optical axis, k isthe conic coefficient, c=1/R where R is the radius of curvature, and aare coefficients given in Table 2 In the equation above as applied toembodiments of a lens assembly disclosed herein, coefficients α₁ and α₇are zero. Note that the maximum value of r “max r”=Diameter/2. Also notethat Table 1 (and in Tables 3 and 5 below), the distances betweenvarious elements (and/or surfaces) are marked “Lmn” (where m refers tothe lens element number, n=1 refers to the element thickness and n=2refers to the air gap to the next element) and are measured on theoptical axis z, wherein the stop is at z=0. Each number is measured fromthe previous surface. Thus, the first distance −0.466 mm is measuredfrom the stop to surface 102 a, the distance L11 from surface 102 a tosurface 102 b (i.e. the thickness of first lens element 102) is 0.894mm, the gap L12 between surfaces 102 b and 104 a is 0.020 mm, thedistance L21 between surfaces 104 a and 104 b (i.e. thickness d2 ofsecond lens element 104) is 0.246 mm, etc. Also, L21=d₂ and L51=d₅. L11for lens element 102 is indicated in FIG. 1A. Also indicated in FIG. 1Ais a width L1 e of a flat circumferential edge (or surface) of lenselement 102. L11 and L1 e are also indicated for each of first lenselements 202 and 302 in, respectively, embodiments 200 (FIG. 2A) and 300(FIG. 3A).

TABLE 1 Radius R Distances Diameter # Comment [mm] [mm] Nd/Vd [mm] 1Stop Infinite −0.466 2.4 2 L11 1.5800 0.894 1.5345/57.095 2.5 3 L12−11.2003 0.020 2.4 4 L21 33.8670 0.246 1.63549/23.91  2.2 5 L22 3.22810.449 1.9 6 L31 −12.2843 0.290 1.5345/57.095 1.9 7 L32 7.7138 2.020 1.88 L41 −2.3755 0.597 1.63549/23.91  3.3 9 L42 −1.8801 0.068 3.6 10 L51−1.8100 0.293 1.5345/57.095 3.9 11 L52 −5.2768 0.617 4.3 12 WindowInfinite 0.210 1.5168/64.17  3.0 13 Infinite 0.200 3.0

TABLE 2 Conic # coefficient k α₂ α₃ α₄ α₅ α₆ 2 −0.4668 7.9218E−032.3146E−02 −3.3436E−02 2.3650E−02 −9.2437E−03 3 −9.8525 2.0102E−022.0647E−04 7.4394E−03 −1.7529E−02 4.5206E−03 4 10.7569 −1.9248E−038.6003E−02 1.1676E−02 −4.0607E−02 1.3545E−02 5 1.4395 5.1029E−032.4578E−01 −1.7734E−01 2.9848E−01 −1.3320E−01 6 0.0000 2.1629E−014.0134E−02 1.3615E−02 2.5914E−03 −1.2292E−02 7 −9.8953 2.3297E−018.2917E−02 −1.2725E−01 1.5691E−01 −5.9624E−02 8 0.9938 −1.3522E−02−7.0395E−03 1.4569E−02 −1.5336E−02 4.3707E−03 9 −6.8097 −1.0654E−011.2933E−02 2.9548E−04 −1.8317E−03 5.0111E−04 10 −7.3161 −1.8636E−018.3105E−02 −1.8632E−02 2.4012E−03 −1.2816E−04 11 0.0000 −1.1927E−017.0245E−02 −2.0735E−02 2.6418E−03 −1.1576E−04Embodiment 100 provides a field of view (FOV) of 44 degrees, withEFL=6.90 mm, F#=2.80 and TTL of 5.904 mm. Thus and advantageously, theratio TTL/EFL=0.855. Advantageously, the Abbe number of the first, thirdand fifth lens element is 57.095. Advantageously, the first air gapbetween lens elements 102 and 104 (the gap between surfaces 102 b and104 a) has a thickness (0.020 mm) which is less than a tenth ofthickness d₂ (0.246 mm). Advantageously, the Abbe number of the secondand fourth lens elements is 23.91. Advantageously, the third air gapbetween lens elements 106 and 108 has a thickness (2.020 mm) greaterthan TTL/5 (5.904/5 mm). Advantageously, the fourth air gap between lenselements 108 and 110 has a thickness (0.068 mm) which is smaller than1.5d₅ (0.4395 mm).

The focal length (in mm) of each lens element in embodiment 100 is asfollows: f1=2.645, f2=−5.578, f3=−8.784, f4=9.550 and f5=−5.290. Thecondition 1.2×|f3|>|f2|<1.5×f1 is clearly satisfied, as1.2×8.787>5.578>1.5×2.645. f1 also fulfills the condition f1<TTL/2, as2.645<2.952.

Using the data from row #2 in Tables 1 and 2, L1 e in lens element 102equals 0.297 mm, yielding a center-to-edge thickness ratio L11/L1 e of3.01.

FIG. 2A shows a second embodiment of an optical lens system disclosedherein and marked 200. FIG. 2B shows the MTF vs. focus shift of theentire optical lens system for various fields in embodiment 200. FIG. 2Cshows the distortion +Y in percent vs. field. Embodiment 200 comprisesin order from an object side to an image side: an optional stop 201; afirst plastic lens element 202 with positive refractive power having aconvex object-side surface 202 a and a convex or concave image-sidesurface 202 b; a second glass lens element 204 with negative refractivepower, having a meniscus convex object-side surface 204 a, with an imageside surface marked 204 b; a third plastic lens element 206 withnegative refractive power having a concave object-side surface 206 awith an inflection point and a concave image-side surface 206 b; afourth plastic lens element 208 with positive refractive power having apositive meniscus, with a concave object-side surface marked 208 a andan image-side surface marked 208 b; and a fifth plastic lens element 210with negative refractive power having a negative meniscus, with aconcave object-side surface marked 110 a and an image-side surfacemarked 210 b. The optical lens system further comprises an optionalglass window 212 disposed between the image-side surface 210 b of fifthlens element 210 and an image plane 214 for image formation of anobject.

In embodiment 200, all lens element surfaces are aspheric. Detailedoptical data is given in Table 3, and the aspheric surface data is givenin Table 4, wherein the markings and units are the same as in,respectively, Tables 1 and 2. The equation of the aspheric surfaceprofiles is the same as for embodiment 100.

TABLE 3 Radius R Distances Diameter # Comment [mm] [mm] Nd/Vd [mm] 1Stop Infinite −0.592 2.5 2 L11 1.5457 0.898 1.53463/56.18 2.6 3 L12−127.7249 0.129 2.6 4 L21 6.6065 0.251 1.91266/20.65 2.1 5 L22 2.80900.443 1.8 6 L31 9.6183 0.293 1.53463/56.18 1.8 7 L32 3.4694 1.766 1.7 8L41 −2.6432 0.696 1.632445/23.35  3.2 9 L42 −1.8663 0.106 3.6 10 L51−1.4933 0.330 1.53463/56.18 3.9 11 L52 −4.1588 0.649 4.3 12 WindowInfinite 0.210  1.5168/64.17 5.4 13 Infinite 0.130 5.5

TABLE 4 Conic # coefficient k α₂ α₃ α₄ α₅ α₆ 2 0.0000 −2.7367E−032.8779E−04 −4.3661E−03 3.0069E−03 −1.2282E−03 3 −10.0119 4.0790E−02−1.8379E−02 2.2562E−02 −1.7706E−02 4.9640E−03 4 10.0220 4.6151E−025.8320E−02 −2.0919E−02 −1.2846E−02 8.8283E−03 5 7.2902 3.6028E−021.1436E−01 −1.9022E−02 4.7992E−03 −3.4079E−03 6 0.0000 1.6639E−015.6754E−02 −1.2238E−02 −1.8648E−02 1.9292E−02 7 8.1261 1.5353E−018.1427E−02 −1.5773E−01 1.5303E−01 −4.6064E−02 8 0.0000 −3.2628E−021.9535E−02 −1.6716E−02 −2.0132E−03 2.0112E−03 9 0.0000 1.5173E−02−1.2252E−02 3.3611E−03 −2.5303E−03 8.4038E−04 10 −4.7688 −1.4736E−017.6335E−02 −2.5539E−02 5.5897E−03 −5.0290E−04 11 0.00E+00 −8.3741E−024.2660E−02 −8.4866E−03 1.2183E−04 7.2785E−05Embodiment 200 provides a FOV of 43.48 degrees, with EFL=7 mm, F#=2.86and TTL=5.90 mm. Thus and advantageously, the ratio TTL/EFL=0.843.Advantageously, the Abbe number of the first, third and fifth lenselements is 56.18. The first air gap between lens elements 202 and 204has a thickness (0.129 mm) which is about half the thickness d₂ (0.251mm). Advantageously, the Abbe number of the second lens element is 20.65and of the fourth lens element is 23.35. Advantageously, the third airgap between lens elements 206 and 208 has a thickness (1.766 mm) greaterthan TTL/5 (5.904/5 mm). Advantageously, the fourth air gap between lenselements 208 and 210 has a thickness (0.106 mm) which is less than1.5×d₅ (0.495 mm).

The focal length (in mm) of each lens element in embodiment 200 is asfollows: f1=2.851, f2=−5.468, f3=−10.279, f4=7.368 and f5=−4.536. Thecondition 1.2×|f3|>|f2|<1.5×f1 is clearly satisfied, as1.2×10.279>5.468>1.5×2.851. f1 also fulfills the condition f1<TTL/2, as2.851<2.950.

Using the data from row #2 in Tables 3 and 4, L1 e in lens element 202equals 0.308 mm, yielding a center-to-edge thickness ratio L11/L1 e of2.916.

FIG. 3A shows a third embodiment of an optical lens system disclosedherein and marked 300. FIG. 3B shows the MTF vs. focus shift of theentire optical lens system for various fields in embodiment 300. FIG. 3Cshows the distortion +Y in percent vs. field. Embodiment 300 comprisesin order from an object side to an image side: an optional stop 301; afirst glass lens element 302 with positive refractive power having aconvex object-side surface 302 a and a convex or concave image-sidesurface 302 b; a second plastic lens element 204 with negativerefractive power, having a meniscus convex object-side surface 304 a,with an image side surface marked 304 b; a third plastic lens element306 with negative refractive power having a concave object-side surface306 a with an inflection point and a concave image-side surface 306 b; afourth plastic lens element 308 with positive refractive power having apositive meniscus, with a concave object-side surface marked 308 a andan image-side surface marked 308 b; and a fifth plastic lens element 310with negative refractive power having a negative meniscus, with aconcave object-side surface marked 310 a and an image-side surfacemarked 310 b. The optical lens system further comprises an optionalglass window 312 disposed between the image-side surface 310 b of fifthlens element 310 and an image plane 314 for image formation of anobject.

In embodiment 300, all lens element surfaces are aspheric. Detailedoptical data is given in Table 5, and the aspheric surface data is givenin Table 6, wherein the markings and units are the same as in,respectively, Tables 1 and 2. The equation of the aspheric surfaceprofiles is the same as for embodiments 100 and 200.

TABLE 5 Radius R Distances Diameter # Comment [mm] [mm] Nd/Vd [mm] 1Stop Infinite −0.38 2.4 2 L11 1.5127 0.919 1.5148/63.1 2.5 3 L12−13.3831 0.029 2.3 4 L21 8.4411 0.254 1.63549/23.91 2.1 5 L22 2.61810.426 1.8 6 L31 −17.9618 0.265  1.5345/57.09 1.8 7 L32 4.5841 1.998 1.78 L41 −2.8827 0.514 1.63549/23.91 3.4 9 L42 −1.9771 0.121 3.7 10 L51−1.8665 0.431  1.5345/57.09 4.0 11 L52 −6.3670 0.538 4.4 12 WindowInfinite 0.210  1.5168/64.17 3.0 13 Infinite 0.200 3.0

TABLE 6 Conic # coefficient k α₂ α₃ α₄ α₅ α₆ 2 −0.534 1.3253E−022.3699E−02 −2.8501E−02 1.7853E−02 −4.0314E−03 3 −13.473 3.0077E−024.7972E−03 1.4475E−02 −1.8490E−02 4.3565E−03 4 −10.132 7.0372E−041.1328E−01 1.2346E−03 −4.2655E−02 8.8625E−03 5 5.180 −1.9210E−032.3799E−01 −8.8055E−02 2.1447E−01 −1.2702E−01 6 0.000 2.6780E−011.8129E−02 −1.7323E−02 3.7372E−02 −2.1356E−02 7 10.037 2.7660E−01−1.0291E−02 −6.0955E−02 7.5235E−02 −1.6521E−02 8 1.703 2.6462E−02−1.2633E−02 −4.7724E−04 −3.2762E−03 1.6551E−03 9 −1.456 5.7704E−03−1.8826E−02 5.1593E−03 −2.9999E−03 8.0685E−04 10 −6.511 −2.1699E−011.3692E−01 −4.2629E−02 6.8371E−03 −4.1415E−04 11 0.000 −1.5120E−018.6614E−02 −2.3324E−02 2.7361E−03 −1.1236E−04Embodiment 300 provides a FOV of 44 degrees, EFL=6.84 mm, F#=2.80 andTTL=5.904 mm. Thus and advantageously, the ratio TTL/EFL=0.863.Advantageously, the Abbe number of the first lens element is 63.1, andof the third and fifth lens elements is 57.09. The first air gap betweenlens elements 302 and 304 has a thickness (0.029 mm) which is about1/10^(th) the thickness d₂ (0.254 mm). Advantageously, the Abbe numberof the second and fourth lens elements is 23.91. Advantageously, thethird air gap between lens elements 306 and 308 has a thickness (1.998mm) greater than TTL/5 (5.904/5 mm). Advantageously, the fourth air gapbetween lens elements 208 and 210 has a thickness (0.121 mm) which isless than 1.5d₅ (0.6465 mm).

The focal length (in mm) of each lens element in embodiment 300 is asfollows: f1=2.687, f2=−6.016, f3=−6.777, f4=8.026 and f5=−5.090. Thecondition 1.2×|f3|>|f2|<1.5×f1 is clearly satisfied, as1.2×6.777>6.016>1.5×2.687. f1 also fulfills the condition f1<TTL/2, as2.687<2.952.

Using the data from row #2 in Tables 5 and 6, L1 e in lens element 302equals 0.298 mm, yielding a center-to-edge thickness ratio L11/L1 e of3.08.

While this disclosure has been described in terms of certain embodimentsand generally associated methods, alterations and permutations of theembodiments and methods will be apparent to those skilled in the art.The disclosure is to be understood as not limited by the specificembodiments described herein, but only by the scope of the appendedclaims.

What is claimed is:
 1. A lens assembly, comprising: a plurality of lenselements arranged along an optical axis and spaced apart by respectivespaces, wherein the lens assembly has an effective focal length (EFL), atotal track length (TTL) of 6.5 millimeters or less and a ratioTTL/EFL<1.0, wherein the plurality of lens elements includes, in orderfrom an object side to an image side, a first group comprising lenselements L₁ _(_) ₁, L₁ _(_) ₂ and L₁ _(_) ₃ with respective focallengths f₁ _(_) ₁, f₁ _(_) ₂ and f₁ _(_) ₃ and a second group comprisinglens elements L₂ _(_) ₁ and L₂ _(_) ₂, wherein the first and secondgroups of lens elements are separated by a gap that is larger than twiceany other gap between lens elements, wherein lens element L₁ _(_) ₁ haspositive refractive power and lens element L₁ _(_) ₂ has negativerefractive power and wherein lens elements L₂ _(_) ₁ and L₂ _(_) ₂ haveopposite refractive powers.
 2. The lens assembly of claim 1, wherein theTTL is equal or smaller than 6.0 mm and wherein the lens assembly has af-number F#<2.9.
 3. The lens assembly of claim 1, wherein the TTL isequal or smaller than 6.0 mm and wherein lens element L₁ _(_) ₁ has animage-side surface diameter between 2.3 mm and 2.5 mm.
 4. The lensassembly of claim 1, wherein f₁ _(_) ₁<TTL/2.
 5. The lens assembly ofclaim 1, wherein the lens assembly has a f-number F#<2.9.
 6. The lensassembly of claim 5, wherein lens element L₁ _(_) ₁ has a concaveimage-side surface.
 7. The lens assembly of claim 1, wherein the lensassembly has a f-number F#=2.
 8. The lens assembly of claim 5, whereinlens element L₁ _(_) ₁ has a convex image-side surface.
 9. The lensassembly of claim 1, wherein 1.2×|f₁ _(_) ₃|>1.5×f₁ _(_) ₁.
 10. The lensassembly of claim 1, wherein 1.2×|f₁ _(_) ₃|>|f₁ _(_) ₂|>1.5×f₁ _(_) ₁.11. The lens assembly of claim 1, wherein a combined power of lenselements L₁ _(_) ₂ and L₁ _(_) ₃ is negative.
 12. The lens assembly ofclaim 1, wherein L₁ _(_) ₃ has negative refractive power.
 13. The lensassembly of claim 1, wherein the gap between lens elements L₂ _(_) ₁ andL₂ _(_) ₂ is smaller than 1.5×d₂ _(_) ₂, where d₂ _(_) ₂ is a thicknessof lens element L₂ _(_) ₂ along the optical axis.
 14. The lens assemblyof claim 1, wherein lens elements L₂ _(_) ₁ and L₂ _(_) ₂ are separatedby a gap smaller than TTL/20.
 15. The lens assembly of claim 1, whereinL₂ _(_) ₁ and L₂ _(_) ₂ are made of different lens materials havingdifferent Abbe numbers, such that one lens element has Abbe number thatis smaller than 30 and the other lens element has an Abbe number that islarger than
 50. 16. The lens assembly of claim 2, wherein the lensassembly further includes a ratio between a largest optical axisthickness L11 and a circumferential edge thickness Lle of lens elementL₁₁ of L11/L1 e<3.
 17. A lens assembly, comprising a plurality of lenselements arranged along an optical axis and spaced apart by respectivespaces, wherein the lens assembly has an effective focal length (EFL),wherein a lens system that includes the lens assembly plus a windowpositioned between the plurality of lens elements and an image plane hasa total track length (TTL) of 6.5 millimeters or less, wherein a ratioTTL/EFL <1.0, wherein the plurality of lens elements includes, in orderfrom an object side to an image side, a first group comprising lenselements L₁ _(_) ₁, L₁ _(_) ₂ and L₁ _(_) ₃ with respective focallengths f₁ _(_) ₂ and f₁ _(_) ₃, and a second group comprising lenselements L₂ _(_) ₁ and L₂ _(_) ₂, wherein lens element L₁ _(_) ₁ haspositive refractive power and lens element L₁ _(_) ₂ has negativerefractive power, wherein 1.2×|f₁ _(_) ₃|>|f₁ _(_) ₂|>1.5×f₁ _(_) ₁ andwherein lens elements L₂ _(_) ₁ and L₂ _(_) ₂ have opposite refractivepowers.
 18. The lens assembly of claim 17, wherein the TTL is equal orsmaller than 6.0 mm and wherein the lens assembly has a f-number F#<2.9.19. The lens assembly of claim 17, wherein the TTL is equal or smallerthan 6.0 mm and wherein lens element L₁ _(_) ₁ has an image-side surfacediameter between 2.3 mm and 2.5 mm.
 20. The lens assembly of claim 17,wherein f₁ _(_) ₁<TTL/2.
 21. The lens assembly of claim 17, wherein thelens assembly has a f-number F#<2.9.
 22. The lens assembly of claim 21,wherein lens element L₁ _(_) ₁ has a concave image-side surface.
 23. Thelens assembly of claim 17, wherein the lens assembly has a f-numberF#=2.8.
 24. The lens assembly of claim 21, wherein lens element L₁ _(_)₁ has a convex image-side surface.
 25. The lens assembly of claim 17,wherein a combined power of lens elements L₁ _(_) ₂ and L₁ _(_) ₃ isnegative.
 26. The lens assembly of claim 17, wherein L₁ _(_) ₃ hasnegative refractive power.
 27. The lens assembly of claim 17, wherein agap between lens elements L₂ _(_) ₁ and L₂ _(_) ₂ is smaller than1.5×d₅, where d₅ is a thickness of lens element L₂ _(_) ₂ along theoptical axis.
 28. The lens assembly of claim 17, wherein lens elementsL₂ _(_) ₁ and L₂ _(_) ₂, are separated by a gap smaller than TTL/20. 29.The lens assembly of claim 17, wherein L₂ _(_) ₁ and L₂ _(_) ₂ are madeof different lens materials having different Abbe numbers, such that onelens element has an Abbe number that is smaller than 30 and the otherlens element has an Abbe number that is larger than
 50. 30. The lensassembly of claim 18, wherein the lens assembly further includes a ratiobetween a largest optical axis thickness L11 and a circumferential edgethickness L1 e of lens element L₁ _(_) ₁ of L11/L1 e<3.